==========================
Joseph, thank you for your explanination, I had read some where that
diffraction are much easier happen with small aperture size, for
example in short focal lenght lens, I didn't know it also happen so
easy with tele which has a physical large aperture hole size.
==========================
that is a common misconception. the smaller the aperture, the worse
the diffraction is at the aperture itself. once light passes through
an aperture, the diffraction effect causes it to start spreading
out. how much it spreads out depends on how far it travels while
spreading out. a longer lens is focused farther away from the
film plane so the light travels further after passing through the
aperture. when the light spreads out, a point of focused light
spreads out into a disk. This is called the Airy disk and it
reduces resolution in the same way that defocused light rendered
as a disk/circle of confusion does.
If you use the diffraction equation to create a model for the size
of the Airy disk at the film plane, it turns out that if you make
some very mild simplifying assumptions, you get a very good approximation
that is mathematically tractable. In this approximation, focal
length is used as an estimate for distance from lens to film and
the aperture diameter and focal length appear in the equation in a
ratio that get replaced by f-stop (since f-stop = focal length / diameter).
the result of this is that both focal length and aperture diameter drop
out of the expression, and it all reduces to a ver nice formula that
any lens is limited by diffraction at f-stop F to a resolution of
about 1600/F lp/mm.
so a 300mm lens at f/8 and a 20mm lens at f/8 have about the same
diffraction limits.
Note that it is the aerial image of the lens that is limited in this way.
The on-film resolution will be even lower.
Joseph
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